Hello 'cheryl',
You cannot yet offer the item to the underbidder because the first person might unexpectedly pay at the same time as the second person, - so unless you have 2 identical items so that each gets one, you need to finish the transaction with the 'winner'.
After 4 days from when the auction ends you can open an Unpaid Item dispute (hour 96 if you want to count) and with still now word from the winner, close it 4 days after that. You get your selling fees back, the nonpayer gets a strike, neither leaves feedback, and you offer the item to the 'runner up'. Here is a link to tell you all about it:
http://pages.ebay.ca/help/sell/unpaid-items.html